Ethylene/vinyl alcohol copolymers (hereinafter referred to as “EVOHs” for short) obtained by saponifying ethylene/vinyl acetate copolymers or the like are particularly excellent in gas barrier properties against oxygen and so forth, odor retaining capacity, oil resistance and chemical resistance, so that they are adequately used in the form of various moldings such as films, sheets and containers. Further, they have attracted attention also as protective coating materials for metal surfaces, paper, wood and the like. In particular, they are greatly superior in gas barrier properties to other resins and, therefore, are very useful as materials for food packaging films, sheets, laminates, hollow containers and so forth which are required to be effective in inhibiting the contents thereof from being oxidized and in odor retention.
As for the general methods of molding EVOH into films, sheets and the like, the method comprising melt extrusion molding or injection molding of EVOH and the method comprising EVOH film lamination are widely practiced. On the other hand, a method has been proposed which comprises application of a solution of EVOH dissolved in a solvent or an aqueous dispersion of EVOH, followed by drying, since when such is used, relatively thin coat films can be formed and coat films can be formed with ease even on articles complicated in form and shape, for example hollow containers.
However, there are problems: with the increasing concentration, the EVOH solution increases in viscosity and becomes difficult to use, and the solvent is a mixed solvent comprising such an organic solvent as dimethyl sulfoxide or an alcohol and water, so that the working environment may be deteriorated by the organic solvent evaporating in the coat film formation process and an apparatus for organic solvent recovery is required to an economical disadvantage. On the contrary, the method comprising application of an aqueous dispersion of EVOH uses water as the dispersion medium and therefore is considered and expected to be advantageous from the working environment and economical viewpoint.
Known as the method of producing aqueous EVOH dispersions are, among others, the method comprising dissolving, with heating, EVOH in the presence of an alcohol, water and a surfactant, followed by emulsifying/dispersing while removing the alcohol (Patent Document 1), the method comprising dissolving EVOH in a hydrophilic solvent, emulsifying/dispersing the EVOH in a precipitating agent, collecting the aggregate by filtration and redispersing the same using a basic substance (Patent Document 2) and the method comprising emulsifying/dispersing a copolymer in which the EVOH component and an ionic group-containing component are bound together in a block—or graft-like manner and which is insoluble in water at ordinary temperature (Patent Document 3). However, the aqueous dispersions obtainable are low in concentration and insufficient in storage stability.
Also known is a method which comprises concentrating the resin component by centrifugation in the course of producing aqueous EVOH dispersions (Patent Document 4), for instance, however, the process is complicated and uneconomical, and the aqueous dispersion obtained cannot be said to be satisfactory in long-term storage stability.
Further, a method comprising melt-mixing EVOH with a dispersing agent using a twin-screw extruder, followed by dispersion in water (Patent Document 5), for instance, is known, however, there is a problem that the particles in the aqueous dispersion obtained are great in particle diameter.    Patent Document 1: Japanese Kokai Publication Sho-54-101844    Patent Document 2: Japanese Kokai Publication Sho-56-61430    Patent Document 3: Japanese Kokai Publication Hei-4-225008    Patent Document 4: Japanese Kokai Publication Hei-5-179001    Patent Document 5: Japanese Kokai Publication Hei-11-193340